The St Pancras Brasserie and Champagne Bar by Searcys is as expansive as its name, but ghostly. It is an immense Art Deco restaurant spilling on to an empty platform at the station. When restaurants opened their patios and gardens, I fretted that they would be too busy to be enjoyed: a diner would cling to a square of Astroturf, fearing to sink. But not here: the people have been removed, and they have not returned.

Inside, it is empty if not shuttered: a great, golden brasserie with dark wood, dark leather and pale globes of light. The door to the loo is so tall I imagine they stole the idea from Mr Greedy (and the giant who loved peas). It is a pastiche of the great French interwar brasseries — London is filled with these, as if it knows something we don’t — but because it is in a station it feels more synthetic than pastiche. It is explicitly mad: a space waiting for a giant hen party or the fall of liberal democracy or, ideally, both together. The platform part, the Champagne Bar, is infinitely more thrilling, because it sits under an iron span roof that was the largest in the world when it was built in 1868. That roof is our ceiling; the floor is pale; there are vast plants, trying to invoke jungle; there are no walls (or boundaries).

Behind us is George Gilbert Scott’s insane red-brick gothic revival former Midland Hotel. In front of us are the tracks to Paris, the Midlands and Kent. I think this may be the platform where ghost Harry Potter met ghost Dumbledore over the evil baby body of Lord Voldemort in accidental horcrux form. I don’t know if they then went to Searcys for brunch.

This platform restaurant has booths, a golden bar, and low, squashy brown sofas: the kind it is hard to rise from. I have been a person who would spend £500 on champagne before falling on to a train to Paris, the Midlands or Kent, so it is quite close to being my ideal restaurant. To call it transient doesn’t really do justice to its transience. It is an alcoholic’s restaurant — it is styled to match champagne, with golden curves, and food is an afterthought. It might as well sell Super Noodles, spam or porridge.

Still, they do their best: they offer an international menu, which is sensible — it is an international clientele — but depressing. There is seafood, salad, risotto; lemon tart, lobster bisque, cheese. There is a rib of beef (£69 for two) and a sausage roll (£6.50). I long, at least in theory, to eat a rib of beef on a sofa next to a train above a shopping mall (of course there is a shopping mall), but it is too magical and too early in the day: the possibility is enough. Or there is a whole lemon sole for £23. Who eats lemon sole on a train platform?

It is peculiar to eat in a station when no one is travelling, but this perversity is pleasing, and Soho is full. I like the idea of Paris better than I like Paris — my sister says I always went with the wrong men. We eat good, cold smoked salmon (£12.50) with weird, Mr Greedy-sized lumps of brown bread, a pretty Caesar salad (£16) and an enormous £17 burger with bacon and cheddar and triple-cooked chips. (The more you cook them, the worse they get. It is like eating futons.) It is food for wealthy, generic, alcoholic giants and hor-cruxes. But still, I am captivated. I wonder if the brick-food is designed to keep this balloon brasserie and its madness close to earth.